How Cities Reshape the Evolutionary Path of Urban Wildlife

The northwest corner of Newark Bay is the kind of place comedians have in mind when they mock New Jersey as a cesspool. The grim industrial coast the bay shares with the Passaic River is lined with the hulks of old chemical plants that treated their surroundings like a toilet. The most infamous of these facilities produced nearly a million gallons of Agent Orange, the toxic defoliant whose extensive use during the Vietnam War has caused generations of suffering. The Agent Orange plant discharged unholy amounts of carcinogenic dioxin—so much, in fact, that New Jersey’s governor declared a state of emergency in June 1983. Though the Environmental Protection Agency has announced a $1.4 billion cleanup effort, the waters closest to Newark’s Ironbound neighborhood remain highly contaminated; there are few worse spots in America to go for a swim.

And yet upper Newark Bay is not devoid of life. Beneath its dull green surface teems a population of Atlantic killifish, a silvery topminnow that’s common along the Eastern Seaboard. These fish are virtually indistinguishable from most other members of their species, save for their peculiar ability to thrive in conditions that are lethal to their kin. When killifish plucked from less polluted environments are exposed to dioxin levels like those in the bay, they either fail to reproduce or their offspring die before hatching; their cousins from Newark, by contrast, swim and breed happily in the noxious soup.

Eight years ago, while he was an associate professor at Louisiana State University, an environmental toxicologist named Andrew Whitehead decided to find out what makes Newark’s killifish so tough. He and his research group collected sample fish from an inlet near the city’s airport and began to deconstruct their genomes, sifting through millions of lines of genetic code in search of tiny quirks that might explain the creatures’ immunity to the ravages of dioxin.

In late 2014, two years after having moved to UC Davis, Whitehead zeroed in on the genes linked to the aryl hydrocarbon receptor, a protein that regulates an array of cellular functions. When most adult killi­fish encounter dioxin, this receptor’s signaling pathway revs to life in the hope of metabolizing the chemical invader. But try as it might, the protein can’t break down the insidious substance. Instead of acting as a defense mechanism, the frustrated signaling pathway wreaks havoc during development—causing severe birth defects or death in embryos. “If you inappropriately activate this pathway when your organs are being developed, you’re really hosed,” Whitehead says. But that ugly fate never befalls the Newark Bay killifish because their bodies are wise to dioxin’s cunning; the genes that control their aryl hydrocarbon receptors, which have slightly different DNA sequences than those found in other killifish, lie dormant when confronted by the toxin.

As he explained in a landmark Science paper in 2016, Whitehead and his colleagues also discovered that Newark Bay’s killifish are not alone in using this clever genetic tactic to survive in tainted water. He identified similarly resilient killifish in three other East Coast cities whose estuaries have been befouled by industry: New Bedford, Massachusetts; Bridgeport, Connecticut; and Portsmouth, Virginia. Since killifish never roam far from where they’re born, these resistant populations must have developed the identical tweaks to their genomes without mixing with one another—or, put more plainly, the far-flung fish all evolved in remarkably similar ways in response to the same environmental pressures. This is compelling evidence in favor of the notion that evolution, that most sublime of nature’s engines, is not some chaotic phenomenon but, rather, an orderly one whose outcomes we might be able to predict.

Whitehead’s work on killifish is one of the signature triumphs of urban evolution, an emergent discipline devoted to figuring out why certain animals, plants, and microbes survive or even flourish no matter how much we transform their habitats. Humans rarely give much thought to the creatures that flit or crawl or skitter about our apartment blocks and strip malls, in part because we tend to dismiss them as either ordinary or less than fully wild. But we should instead marvel at how these organisms have managed to keep pace with our relentless drive to build and cluster in cities. Rather than wilt away as Homo sapiens have spread forth bearing concrete, bitumen, and steel, a select number of species have developed elegant adaptations to cope with the peculiarities of urban life: more rigid cellular membranes that may ward off heat, digestive systems that can absorb sugary garbage, altered limbs and torsos that enhance agility atop asphalt or in runoff-fattened streams.

Whitehead and his colleagues, many of whom are at the dawn of their careers, are now beginning to pinpoint the subtle genetic changes that underlie these novel traits. Their sleuthing promises to solve a conundrum that has vexed biologists for 160 years, and in the process reveal how we might be able to manipulate evolution to make the world’s cities—projected to be home to two-thirds of humanity by 2050—resilient enough to endure the catastrophes that are coming their way.